* Roll engine to b6df7a637498ca9beda1fa9cd7210e3202ea599f.
Changes since last roll:
```
b6df7a637 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (#4771)
a6764dbd5 Add sources for Fuchsia target. (#4763)
2d5900615 [fuchsia] Remove unused header file. (#4769)
9717063b7 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)" (#4768)
9a9814312 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)
e74e8b35c [async] Update includes of async headers to new path (#4760)
e2c4b2760 Use Dart 2 camel case constants in the engine Dart libraries (#4766)
9c1e48434 Updates for Fuchsia roll. (#4765)
14c940e27 Switch from fxl::Mutex to std::mutex (#4764)
debf82c0b Roll Garnet (#4759)
5bffdefbb Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)
```
* Update the following packages
- dart-petitparser to 1.7.5
- dart-xml to 3.0.3
- image to 1.1.30
These updates fix some of the strong mode errors we were encountering
while running flutter tests.
This fixes the error handling for the packaging script so that it will properly report a failure exit code when it can't find the executable that it's looking for.
Added a test too.
Fixed the real repaint problem the Slider had (missing addListener), and added tests for it.
Added GlobalKey reparenting test.
Added the ability for the value indicator to slide left and right to avoid falling off the edge of the screen.
It only shifts as much as it can without deforming, but even at large text scales, that is enough to keep the text on the screen.
Updated the formatting on theme_data.dart and others to use longer line length.
Also, removed a color tween that faded the value indicator in as it scaled, since that wasn't to spec.
* Update dependencies with forced upgrade.
Now that https://github.com/flutter/flutter/issues/15261 is fixed, do the update with upgrade.
* Update test package use following upgrade
* Replace addOption(allowMultiple) with addMultiOption
* add fromSpan constructor to Text widget and simple smoke test:
* change fromSpan to rich, clean up comments
* 'directly from' to 'with'
* make sure text styles are applied to either data or textspan. add diagnostic properties for span
* add expectation for text style to test case
* some work on diagnostics and docs
Make flutter test support the --track-widget-creation flag.
Add widget creation location tests. Tests are skipped when
--track-widget-creation flag is not passed.
* Create frontend compiler in `flutter test` lazily.
This is needed to avoid unwarranted runtime dependencies on Artifacts,
for example when in non-preview-dart-2 mode.
Refactor the compilation-related code into _Compiler class.
* RenderWrap.performLayout takes should take spacing into consideration when deciding to move to next row/axis
* fix spacing in text
* remove dart:developer import
* remove extra line
Some file I/O is piped from OS processes and never completes,
so without havign a timeout on the recording serialzation, we
end up waiting forever on that I/O to complete.
